1. Field of the Invention
The present invention relates to the field of homogenizing and shaping a beam of light, such as laser light, using a diffractive optical element. In particular, the invention provides a method for designing a pattern of diffracting and phase-shifting elements that form a selected image when illuminated with a laser. The invention further provides an efficient method for compensating for the partial spatial coherence of a laser beam in the pattern design.
2. Discussion of the Prior Art
Lasers have been applied to many materials processing operations, including ablation of plastic films, welding and soldering of metals, cutting and marking of both metals and non-metals and annealing of materials including semiconductors. The latter application includes liquid and solid phase epitaxy, crystalinity growth, activation and diffusion of dopants, and elimination of lattice defects. The alternative to laser annealing of semiconductors, furnace annealing, requires heating semiconductors at temperatures between 500 degrees and 1000 degrees for prolonged periods, which is neither convenient nor as effective as laser annealing.
Ultraviolet (UV) excimer lasers have recently been applied as semiconductor processing tools. Typical applications have included semiconductor annealing, microphotolithography, photodeposition, laser-induced chemical vapor deposition (CVD), gas immersion laser doping (GILDing), micromachining, and several other processes. In nearly all of these applications, laser output beam intensity profile uniformity is of paramount importance. Hereafter, the term "beam uniformity" will be employed to refer to beam intensity profile uniformity. Present discharge UV excimer laser technology does not produce laser output beams having adequate uniformity while maintaining required laser output energy.
Available commercial UV excimer lasers do not maintain an adequate level of uniformity over the beam area to ensure sufficient uniform energy density. Further complicating this problem is the presence of occasional and essentially unpredictable changes in laser output beam uniformity on a shot-to-shot basis. Also, as feature sizes (such as semiconductor structures and tolerances) become smaller, the requirements for laser output beam uniformity become more severe.
Optical integrators have been used to homogenize a beam in various types of illumination systems. In most optical integrators, the homogenization of the input beam occurs in one of two ways. Optical integration techniques typically involve either some kind of randomization of the laser output beam (in phase or amplitude) or separation and overlapping of numerous beam segments. The input beam can either be scrambled by a diffuser (a set of lenses with partially overlapping outputs, random phase shift masks, or echelons) or by multiple scatterings in a tube much like a kaleidoscope. Alternatively, the input beam can be broken apart into segments and these segments then imaged on top of one another to average out fluctuations in beam intensity.
Laser beams often do not have a uniform distribution of intensity throughout their cross-section. This is true of most laser sources. The intensity distribution of many laser beams is described, for instance, by a bell shaped curve (Gaussian profile) which is rotationally symmetrical to the direction and spread of the beam. In the case of the so-called unstable resonators, the intensity of the laser beam is sometimes characterized by a hole in the middle of the beam cross-section. Beam profile variations may also be due to anode to cathode variations in an excimer laser. Intensity peaks, so-called "hot spots", frequently occur both with pulsed and continuous laser sources. These are limited areas in the beam cross-section where the intensity of the laser beam is much greater than in the remainder of the area. Such hot spots can appear either at certain locations or they may move within the cross-section of the beam.
This irregular distribution of the beam intensity throughout the beam cross-section and solid angle is disadvantageous for a variety of applications of laser beams. The irregular intensity distributions or intensity peaks and inhomogeneous or unsymetrical angular energy distributions can degrade laser processing, for instance, when pieces of a large area are to be ablated or processed using laser beams.
In many applications, not only must the irregular laser intensity profile be smoothed out before a useful beam can be provided, but the beam must be spread and shaped into a preselected distribution providing equal (or predetermined) intensity throughout the beam distribution area. For example, it may be desirable to use laser ablation to form perforations in a sheet of plastic film. It is desirable to be able to form hundreds of high precision holes, vias or perforations in a given area at one time, using a beam of laser energy that passes through a mask defining the locations of the perforations. In order to obtain the required precision and uniformity of feature sizes on the final product, a substantially equal amount of laser energy must be applied through each hole in the mask. To accomplish this, a highly uniform laser beam must be applied to the areas of the mask where the holes are located. It is simultaneously desirable to conserve energy by applying the laser energy only to the portions of the mask where the holes are located. In a presently preferred embodiment, such holes are located along a pair of long narrow strips of area (as shown in FIG. 1). It is therefore desirable to direct or focus the laser energy into a pair of long narrow beams, each having a highly uniform intensity profile throughout the length and breadth of the illuminated field.
U.S. Pat. Nos. 4,733,944 and 5,414,559 disclose refractive optical devices for homogenizing a light beam. These systems use refractive elements to generate homogenized images having selected sizes and aspect ratios. U.S. Pat. No. 4,733,944 discloses an apparatus for producing a rectangular homogenized beam suitable for use in semiconductor processing. The method disclosed in that patent does not, however, provide a technique for forming an homogenized image having an arbitrary size and shape, as is required by many current applications. Furthermore, the finite spatial coherence (or non-zero beam divergence) of the incident beam contributes to blurring in all homogenizers, which is not compensated for in most prior art technologies including those described herein.
U.S. Pat. No. 5,414,559 provides a refractive arrangement for generating spatially separate homogenized illumination fields from a laser beam. The approach illustrated in that patent requires an arrangement of complex refractive optical elements that must be specially designed to provide the illumination field pattern that is desired. This device is directed to the same problem as the present invention, that is to form two elongated images or illuminated fields having a uniform intensity distribution, but it is expensive and cannot be extended to more general illumination field patterns. Furthermore, it does not lend itself to correction for finite spatial coherence or modification for numerical aperture distribution.
U.K. Patent Application GB 2,278,458 discloses a laser beam homogenization device that comprises a phase zone plate array having a random two-dimensional array of close-packed diffracting Fresnel-type zone plates. This device, when used in conjunction with a principal focusing lens, is used to give a laser beam having a non-uniform intensity profile a more uniform intensity profile before directing it to a workpiece. This device does not effect the shape or size of the image created by the laser beam on the workpiece.
U.S. Pat. No. 4,475,027 discloses an optical beam homogenizer that divides and redirects a beam to provide uniform irradiation to a plane surface, using a reflective arrangement of segmented mirrors. This patent also discloses a refractive beam homogenizer, as illustrated in FIG. 5 of the '027 patent. Both of the reflective and refractive techniques disclosed in the '027 patent provide only a square or rectangular image. This approach does not provide the ability to form the illumination field into an arbitrary pre-selected shape, as is required in many applications.
The use of computer generated holograms for beam shaping is described in "Design of computer-generated beam-shaping holograms by iterative finite-element mesh adaption", T. Dresel, et al, Applied Optics 35:35, p. 6865-74, Dec. 10, 1996. This reference describes a numerical approach based on intuitive finite-element mesh adaptation that permits the design of appropriate phase functions for the task of focusing a laser beam into two-dimensional reconstruction patterns. Both the hologram aperture and the reconstruction pattern are covered by mesh mappings, and an intuitive procedure designs meshes with intensities equally distributed over the constituting elements.
It should be noted that performance of all homogenizers based on overlap of beam segments, including the prior art described above, in terms of uniformity of the resultant illumination field, is directly dependent of input beam symmetry, alignment of the beam with respect to the entrance aperture of the homogenizer, and laser pointing stability. If a perfectly symmetric beam is used, but misaligned relative to the homogenizer, then the resulting illumination field will not be completely uniform. Likewise, if an asymmetric beam intensity distribution is applied to the homogenizer, then the illumination field will also not be completely uniform. Any continuous mathematical function, including a laser beam intensity distribution, can be reduced to a sum of a symmetric and an anti-symmetric contributions. The symmetric portion (with respect to the center of the homogenizer entrance aperture) will generate a perfectly uniform output beam as a result of the incoherent superposition of its constituent beamlets. The anti-symmetric portion, usually much smaller in overall magnitude, will contribute to the lack of uniformity of the output beam. The greater the number of homogenizer elements, the smaller the overall variation of the anti-symmetric contribution.
Laser processing systems can also show variations in the uniformity of the ablated results, even if homogenizer performance and laser beam properties are ideal. For systems that are designed to be symmetric about the optical axis of the system (i.e., comprised of spherical optics, cylindrical optics, and aspheres symmetric about the optic axis), experience has shown that the variation in ablation results across the field of view of the imaging lens may take the shape of a "smile" or a "frown." For instance, if a long row of holes is drilled across the field of view of the imaging lens, the hole in the center may be largest (or smallest) with a symmetrical variation across the workpiece. This variation can be due to chromatic aberrations in the imaging lens, aging effects of the various optics, etc. Variations in a system that is not well aligned, or that has a rather non-ideal laser beam on its input, need not have this characteristic "smile" or "frown" variation, but it is the experience of the inventors that this is the most commonly occurring variation in a well behaved system. Homogenizers in the prior art, as described above, lack the flexibility of providing a "process correction" by changing the illumination field intensity to compensate for these effects.
The prior art described above does not provide an economical, efficient and flexible method for homogenizing a laser beam and providing an arbitrary selected illumination pattern, while providing compensation for the non-ideal nature of the beam. It is desirable to provide a homogenizing optical element that compensates for irregularities in a beam, including its lack of symmetry and partially spatially coherent nature, and to provide a method for designing and manufacturing such an element.